Generally, an automatic transmission includes a plurality of planetary gear sets, and a plurality of hydraulically driven frictional engagement elements (clutches and brakes) for switching a power transmission path by each of the planetary gear sets. Further, various ranges such as a forward range and a reverse range are attained by selective engagement of a specific one of the frictional engagement elements.
In particular, in an automatic transmission to be mounted in a vehicle, in view of the aspect of securing vehicular safety, it is required to determine a range of the automatic transmission on the basis of an actual state within the automatic transmission. For instance, in Japanese Unexamined Patent Publication No. 2004-232732, a currently attained range of an automatic transmission is determined on the basis of a hydraulic pattern to be supplied to each of the frictional engagement elements of the automatic transmission.
In an actual automatic transmission, however, a frictional engagement element may not be operated as intended by a hydraulic pattern due to a defect of a hydraulic circuit or a frictional engagement element. In this case, if a range is determined on the basis of hydraulic pattern of a frictional engagement element as disclosed in Japanese Unexamined Patent Publication No. 2004-232732, it may be impossible to handle the aforementioned matter, and the range may be erroneously determined. In particular, a forward range and a reverse range have such a relationship that the rotational directions of an output shaft are opposite to each other. Therefore, the influence when the range is erroneously determined is serious, and careful determination is required.